A liquid crystal display device generally has a liquid crystal cell and a polarizing plate. A polarizing plate is comprised of a protective film and a polarizing film, and is obtained by a process including: dyeing a polarizing film composed of a polyvinyl alcohol film with iodine, orienting or stretching the resultant film, and laminating a protective film on each of both sides of the thus-oriented film. In a transmissive-type liquid crystal display device, such a polarizing plate is attached to each of the both sides of a liquid crystal cell, and optionally one or more optical compensation film(s) is further provided. In a reflective-type liquid crystal display device, a reflective plate, a liquid crystal cell, one or more optical compensation film(s), and a polarizing plate are arranged in this order. A liquid crystal cell is comprised of a liquid crystalline molecule, two substrates for enclosing the molecule, and electrode layers for applying voltage to the liquid crystalline molecule. Such a liquid crystal cell carries out displaying the state of ON/OFF, depending on difference in the alignment or orientation state of the liquid crystalline molecule. There are proposed display modes, applicable to each of the transmissive-type and reflective-type displays, such as TN (twisted nematic), IPS (in-plane switching), OCB (optically compensatory bend), VA (vertically aligned), and ECB (electrically controlled birefringence) modes.
An optically anisotropic layer is used for various liquid crystal display devices, in order to eliminate image coloration or widen viewing angle. As the optically anisotropic layer, oriented birefringent polymer films have been used. It has been proposed to use an optically anisotropic layer formed of a liquid crystalline molecule on a transparent support, in place of the optically anisotropic layer of the oriented birefringent film. Since liquid crystalline molecules provide various alignment forms, the use of liquid crystalline molecules has made it possible to realize optical properties that are not achievable using conventional oriented birefringent polymer films.
The optical properties of the optically anisotropic layer are determined depending on the optical properties of liquid crystal cells, specifically depending on difference in display mode as mentioned above. The use of liquid crystalline molecules allows the production of optically anisotropic layers having various optical properties depending on various display modes of liquid crystal cells. Rod-like liquid crystalline molecules or discotic liquid crystalline molecules are generally used as liquid crystalline molecules. Optically anisotropic layers using liquid crystalline molecules have already proposed for various display modes.
There is a report of a method of producing an optically anisotropic material in which a low-molecular bifunctional liquid-crystal acrylate compound is used (JP-A-03-014029 (“JP-A” means unexamined published Japanese patent application)). This technique includes aligning a low-molecular bifunctional liquid-crystal acrylate compound or composition in twisted nematic alignment, and then photopolymerizing the compound or composition such that the alignment state would be fixed.
On the other hand, in recent years, it has become essential to form a liquid crystal cell into a thin film in order to reduce the weight and manufacturing costs of liquid crystal display devices. Consequently, it has become necessary to achieve a desired optical anisotropy by the use of a thinner film as an optically anisotropic layer. In order to solve this problem, it is necessary to increase the refractive index anisotropy (Δn) of liquid crystal compounds to form optically anisotropic layers.
Known examples of polymerizable liquid crystal compounds with large Δn include compounds having a tolane moiety in the molecule, as disclosed in JP-A-2002-128742 and JP-A-2002-265475. However, a tolane-series compound has an absorption in a longer wavelength side, and thus it is feared that it may be difficult to proceed radical polymerization of the compound and that the compound may cause coloration of optically anisotropic layers. Further, since a tolane compound is low in stability to light, it is not necessarily preferred in view of film stability.